In the construction of wooden doors, for example for domestic use, it is standard procedure to have a core sub-assembly comprising the upper and lower rails of an inner frame interconnected by an expandable lattice spacer panel of cardboard, to subsequently attach the stiles to the rails and the cardboard lattice to the stiles, and then to put the two outer panels one on each side of the door. One of the time-consuming jobs which is encountered is the opening of the two rails away from one another so as to expand the cardboard core, and the attachment thereof to the stiles. The main object of this invention is to provide a machine which will achieve these results inexpensively and at higher production rates.